Scents of Precious Woods
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Scents of Precious Woods Document Transcript

  • 1. threo and erythro trans-3-isocamphylcyclohexanols as shown lets examine the role of enantioselectivity in odor perception onin Table 1. As will be noted, the (+)-threo enantiomer possesses the enantiomers of 1-hydroxy-1,4,7,7,9-the most desirable odor and strength followed by the erythro pentamethylspiro[4.5]decan-2-one (a patchouli-like odorant),enantiomers, while the (-)-threo enantiomer possesses no Arborone (the odor active component of the popular Iso Esandalwood character. Super®), desmethyl-Arborone, and Georgywood®. In 2005, Kraft and co-workers reported the synthesis and odor evaluation of the enantiomeric forms of 1-hydroxy-1,4,7,7,9- pentamethylspiro[4.5]decan-2-one (which we will hereafter refer to as “spiropatchoulolone”) in which the stereocenters of the odor active (+)-(1S,4R,5R,9S)-enantiomer superimpose well with those of (-)-patchoulol (14). While (+)-spiropatchoulone possesses a powerful woody Table 1. The trans-3-isocamphylcyclohexanols patchouli character (with an odor threshold of 0.067 ng/L in air for the racemate), the ent- form is essentially odorless. In 2006, Hong & Corey (15) reported the first stereospecific syntheses and provided odor evaluations of the enantiomers of Aborone, desmethyl-Arborone and Georgyone. Racemic Arborone, which comprises only ~5 percent of commercial Iso E Super®, has been shown by Fráter et. al. (14) to have an intense warm, woody, ambery character and an odor threshold as low as 5 pg/l (in air) and is the primary contributor to Iso E Super’s odor profile.Other popular synthetic sandalwood odorants (e.g. Javanol®) Hong & Corey have shown that desmethyl-Arborone provideshave primarily been based on materials derived from α- a similar odor profile.campholenic aldehyde. As the enantiomeric versions of thisaldehyde are easily prepared from the (+)- or (-)-α-pineneepoxides, there are many examples of odor evaluations ofenantiomers from materials such as Ebanol (8 diastereomers),where only the (1S,2S,3R)-Ebanol and (1R,2S,3R)-Ebanolhave powerful sandalwood odors (10).PATCHOULIThe powerful woody odor of patchouli is due to primarily to (-)- In the case of Georgywood®, which was introduced intopatchoulol, (+)-norpatchoulenol and pogostol (which comprise fragrances by Givaudan in the late 1990’s (for example –about 30-40 percent, 0.3-0.5 percent and 1.0-2.5 percent of “Golden Moments” by P. Presley, where Georgywood is used atpatchouli oil, respectively). The stereoselective synthesis of (+)- about 5 percent), the enantiomers have been examined by and (–)-patchoulol both Corey’s group (15) and Fráter’s group (17). was achieved at The enantiomeric odor profiles of these three structurally similar Firmenich by Naf et woody odorants appear in Table 2. al. in 1981 and the odor evaluations of the enantiomers carried out (11). “The synthetic,nature-identical (-)- patchoulol exhibits a strong, typicalpatchouli scent with an earthy, slightly camphoraceous,powdery cellar note which is practically indistinguishable fromnatural patchouli alcohol. In contrast to the odour profile ofthe (-)-enantiomer, the unnatural (+)-patchoulol is muchweaker, less characteristic, nearly indefinable and by nomeans reminiscent of patchouli. It might however havea β-santalol odour with a green undertone.”Although a stereospecific synthesis of (+)-norpatchoulenol wasdeveloped by Oppolzer (12, 13) that is also amenable to the Table 2.Other Synthetic Woody Odorantspreparation of the unnatural (-)-enantiomer, the odorcomparison of these appears not to have been carried out. AGARWOODOTHER WOODY ODORANTS Is perhaps the most valued wood for a perfume material in theBefore examining the subject of the key odorants of Agarwood, world. According to statistics the trade in agarwood exceeds a 37Supplement to Chimica Oggi/CHEMISTRY TODAY Vol 24 nr 4 • Chiral technologies
  • 2. The aromatic part of agarwood consists primarily of a complex mixture of oxygenated sequiterpenoids and chromones (20), a number of which appear to contribute to the woody oriental-incense aroma. Three important naturally occurring aroma constituents of agarwood - (+)-Jinkohol II (21), (+)- Karanone (22) and (+)-Dihydrokaranone (23) have been evaluated for their odor properties versus their enantiomers. billion dollars U.S. In fact, agarwood is only the “resinous” The odors of the enantiomeric forms are shown in Table 3. portion of wood from trees belonging to the Aquilaria genus, Thymelaeceae family. At least fifteen species of Aquilaria trees CONCLUSIONS are known to produce agarwood. A whole range of qualities This short overview of the role of chirality on key odorants and products are on the market and prices range from a few responsible for scents of precious woods used in perfumery dollars per kilo for the lowest quality to over thirty thousand US provides clear evidence of enantioselectivity in odor dollars for top quality oil and resinous wood. Aquilaria trees perception. However, as in the case of Jinkohol II, such are native to Asia from Northern India to Vietnam and enantioselectivity is certainly not universal. Nevertheless, Indonesia. Only by cutting trees down and extracting the the potential use of molecular modeling against olfactory valued sections can agarwood be harvested in commercially receptor models (as well as biological work involving the attractive quantities. This has resulted in the rapid demise of odorant activation of olfactory glomerulus using optical Aquilaria in the natural forests of tropical South and Southeast detection fluorescence microscopy) as described by Hong Asia. Several species of Aquilaria are considered endangered and Corey (15), as well as the more classical approach of due to over harvesting. Aquilaria crassna Pierre ex Lecomte is using molecular overlays of new (woody) odorants as used listed as an endangered species in Vietnam, and Aquilari by Kraft (14) provide screening and modeling tools of malaccensis Lam. is listed as endangered by the World promise for odor prediction. For additional reading, the Conservation Union, IUCN and is protected worldwide under articles of Brenna et. al. (24) and Kraft et. al. (25) are the (CITES) convention (although illegal trading is still recommended. prevalent) (18). The healthy wood of Aquilaria trees is white, soft, even- grained, and not scented when freshly cut. Under certain pathological conditions, the heartwood becomes saturated with REFERENCES AND NOTES resin, and eventually becomes hard. The best grade of 1. We thank a referee for providing this insight. agarwood is nearly black and sinks when placed in water. In 2. A. Krotz, G. Helmchen, Tetrahedron Asymmetry, pp. 537-540 (1990); general, agarwood is considered inferior if it is lighter in tone, ibid,. Liebigs Ann. Chem., pp. 601-609 (1994) with diminishing amounts of resin. It was long thought that 3. I. Aulchenko, L. Kheifits, Am. Perf. Cosmet., 85, p. 37 (1970) agar deposits were created as an immune response by the tree, 4. E. Demole, Helv. Chim. Acta, 47(1), pp. 319-338 (1964) the result of an attack by a fungus. But recent experiments by 5. E. Demole, Helv. Chim. Acta, 47(7), pp. 1766-1774 (1964) Blanchette (19) (as part of The Rainforest Project Foundation’s 6. E. Demole, Helv. Chim. Acta, 52(7), pp. 2065-2085 (1969) 7. J. Dorsky, W. Easter, US 3499937 (1970) effort to preserve endangered species of the world’s forests) 8. J. Hall, W. Wiegers, US 4,014,944 (1977) indicate that open wounds subject to aeration can create the 9. M. Emura, T. Toyoda & I. Nishino, JP11-35968 (1999) agarwood resin. Today, as part of this project, several 10. J. Bajgrowicz & G. Fráter, EP 0841318 (1998) Aquilaria plantations in Vietnam, are beginning to produce 11. F. Näf et al., Chim. Acta, 64(5), pp. 1387-1397 (1981) “cultivated” agarwood. 12. W. Oppolzer, R. Snowden, Tetrahedron Letters, 19(37), pp. 3505-3506 (1978) 13. W. Oppolzer, US 4277631 (1981) 14. P. Kraft et al., Eur. J. Org. Chem., 2005(15), pp. 3233-3245 (2005) 15. S. Hong, E. J. Corey, J. Am. Chem. Soc., 128(4), pp. 1346-1352 (2006) 16. C. Nussbaumer et al., Helv. Chim. Acta, 92(7), pp. 1016-1024 (1999) 17. G. Fráter et al., Tetrahedron Asymmetry, 15, pp. 3967-3972 (2004) 18. TRP Agarwood Project Information and Conference Website, http://www.therainforestproject.net/ (accessed April 26, 2006) 19. R. Blanchette, H. van Beek, US 6848211 (2005) 20. B. Lawrence, Perf. Flav., 23, September/October, pp. 62-66 (1998) 21. T. Nagashima, T. Yoshida, US 4444982 (1094) 22. I. Mazakuza, K. Taro, JP 2004231519 (2004) 23. I. Mazakuza, K. Taro, JP 2004189643 (2004) Table 3. Agarwood Odorant Enantiomers 24. E. Brenna et al., Tetrahedron: Asymmetry, 14, pp. 1-42 (2003) 25. P. Kraft et al., Angewandte Chemie, 39(17), pp. 2980-3010 (2000) JOHN C. LEFFINGWELL Leffingwell & Associates 4699 Arbor Hill Rd Canton, GA 30115, USA38 Supplement to Chimica Oggi/CHEMISTRY TODAY Vol 24 nr 4 • Chiral technologies